The invention relates to a device comprising an elongated field-sensitive ferromagnetic layer of which the magnetization is sensitive to an applied magnetic field. Within the concept of the invention xe2x80x98elongatedxe2x80x99 means that layer has an elongated shape i.e. the dimension of the layer in a first direction (the xe2x80x98lengthxe2x80x99) is greater than the dimension (the xe2x80x98widthxe2x80x99) in a direction perpendicular to the first direction. The first direction is commonly called the xe2x80x98longitudinal directionxe2x80x99. The field-sensitive layer may be, for example, (part of) a flux guide or (part of) a magnetoresistive element. The field-sensitive layer may be the second layer of a device comprising a first and a second layer of ferromagnetic material separated by a spacer layer of non-ferromagnetic material, the first and second layer being formed such that the magnetization of the first ferromagnetic layer is maintained in a fixed or pinned direction in the presence of an applied magnetic field, while the magnetization direction of the second ferromagnetic layer is able to change in the presence of an applied magnetic field, the second layer comprising an elongated layer of magnetoresistive material, the device comprising means for directing a sensing current through the elongated layer in a current direction parallel to the longitudinal direction of the layer or perpendicular to the plane of the film. Such a device may be a magnetoresistive sensor or memory element, such as a TMR (Tunnel Magneto Resistance) or GMR (Giant Magneto Resistance) sensor or memory element. The layer of non-magnetic material, which separates the first and second layer of a ferromagnetic material, is often called xe2x80x9cspacer layerxe2x80x9d in the case of a GMR sensor or memory element, and xe2x80x9cbarrier layerxe2x80x9d in the case of a TMR sensor or memory element. In the following text, the more general term xe2x80x9cseparation layerxe2x80x9d will be used in order to indicate the spacer layer or the barrier layer of a GMR or a TMR element, respectively.
A device of the type described in the opening paragraph is known, for example, from U.S. Pat. No. 5,465,185, wherein a sensor is shown. The first layer is also called the xe2x80x98pinned layerxe2x80x99 and the second the xe2x80x98free layerxe2x80x99.
Within the concept of the invention, a xe2x80x98free layerxe2x80x99 is a layer whose magnetization direction is able to change at applied fields with a strength lower than the strength of the field required for changing the magnetization direction of the xe2x80x98pinnedxe2x80x99 layer. Such changing may be a nearly free rotation so as to follow the applied magnetic field such as, for example, in sensors or a changing between two anti-parallel states such as, for example, in memory elements.
It is often preferred that the elongated field-sensitive layer, when used in a magnetoresistive element for GMR devices in which a sensing current is parallel to the longitudinal direction, has a relatively large ratio of the length (l) over the width (w) of the layer, and thus forming a stripe. Such a ratio leads to a large resistance of the stripe, and hence, for a given sense current Isense, a given relative resistance change xcex94R/R and a given sheet resistance R↑, to a large voltage difference between the two extreme states of the element:
xcex94V/V=(l/w)xc2x7xcex94R/Rxc2x7R↑xc2x7Isensexe2x80x83xe2x80x83(1)
However, such devices with a l/w ratio greater than 1, and especially when having a large t(thickness)/w(width) ratio, prove to have a relatively large field that is required to change the magnetization direction of the free layer. In practice this hampers, for example, the use of GMR spin valves as MRAMs (Magnetic Random Access Memories), because large currents would be required to switch the element, leading to the necessity to use a transistor which is much larger than the magnetic element itself. Another example is the application of AMR and GMR sensors for measuring angles, using an elongated, preferably long and narrow stripe shaped MRE (magnetoresistive Element) (i.e. one having a l/w ratio greater than 1) and more preferably having a large t(thickness)/w(width) ratio. A small and preferably zero net magnetic anisotropy is required, as the magnetization of such a sensor should be saturated for all directions of the (moderate) applied field to enable an accurate angle reading to be performed.
The above equation (1) refers to the situation with the current in the plane of the layers. Recently sensor or memory element structures in which the current is directed perpendicular to the plane of the layers have attracted much attention. Examples are spin tunneling structures (a sandwich structure of two ferromagnetic layers with an oxidic tunneling barrier in between). For these structures no large l/w ratio is required from the point; of view of output voltage. However, the need for a large l/w ratio may nevertheless arise in specific sensor (such as read heads) or magnetic element designs.
The problem discussed above becomes particularly serious upon the miniaturization of sensor and memory elements to a scale at which stripe widths enter the nanometer range (w less than 1 xcexcm).
It is an object of the invention to provide a device with a reduced net magnetic anisotropy and/or reduced switching field.
To this end, a device in accordance with the invention is characterized in that the field-sensitive ferromagnetic layer comprises a sandwich structure comprising two magnetic layers, with parallel easy magnetization axes due to the magnetocrystalline anisotropy, and with opposite magnetization directions due to anti-ferromagnetic coupling by a non-magnetic layer 3 separating the two layers 1 and 2, while the magnetization directions due to the magnetocrystalline anisotropy for the two magnetic layers are directed transversely to the longitudinal axis of the field-sensitive ferromagnetic layer.
Terms known to the person of skill in the art are used in the present specification. Some of these are summarized herebelow. The layer of non-magnetic material, which separates the first and second layer of a ferromagnetic material, is often called xe2x80x9cspacer layerxe2x80x9d in the case of a GMR sensor or memory element, and xe2x80x9cbarrier layerxe2x80x9d in the case of a TMR sensor or memory element. In the following text, the more general term xe2x80x9cseparation layerxe2x80x9d will be used in order to indicate the spacer layer or the barrier layer of a GMR or a TMR element, respectively. Magnetocrystalline anisotropy is the dependence of the (free) energy of a ferromagnetic material on the direction of magnetization of the material, resulting from the internal structure in the material. The total magnetic anisotropy of a ferromagnetic body such as a layer is the result of magnetocrystalline anisotropy, plus a contribution from the shape anisotropy of the body. The shape anisotropy is due to the dependence of the total energy in the magnetostatic field due to the magnetization of the body. The external shape of the body defines preferred directions of magnetization of the body. Magnetic anisotropy creates a situation in which the total energy of the body is lowest when the magnetization direction is parallel or antiparallel to one specific direction, or to several directions which are equivalent from the point of view of the symmetry of the body. Such a direction is called an easy axis direction. For layer structures the shape anisotropy is such that the preferred magnetization directions are confined to the plane of the layer or of a stock of layers (a film). This is a situation of application for a number of embodiments in this patent application (i.e.: there is no contribution from magnetocrystalline anisotropy which more than counteracts the shape anisotropy). In addition, the magnetization within the plane of the layer can have one or more easy directions. In a number of embodiments in this patent application the situation is considered in which the magnetocrystalline anisotropy of the magnetic free layer is uniaxial, i.e., in which there is only one preferred easy direction with the plane of the layer (film). This situation can be created, e.g., by growth of the layer (film) in a magnetic field parallel to the required easy axis direction, or by growth of a layer (film) on a substrate on which the magnetic layer (film) develops an internal structure which gives rise to the required uniaxial symmetry.
By patterning a thin ferromagnetic layer (film) in the form of a narrow stripe, using e.g. lithography, shape anisotropy gives rise to a preferred magnetization direction parallel to the length direction of the stripe.
The invention has a number of advantages:
1. The large switching field in conventional devices is caused by a relatively large shape anisotropy.
2. Providing a sandwich structure comprising two ferromagnetic layers, coupled anti-ferromagnetically by a preferably thin non-magnetic layer, the easy magnetization direction for the two magnetic layers being directed perpendicularly to the longitudinal axis of the field-sensitive layer, leads to a decrease of the shape anisotropy field, compared with conventional devices, because the effective saturation magnetization of the pair of anti-ferromagnetically coupled layers is smaller than that for a single layer. The proper expression for the effective shape anisotropy field Hshape,eff for an infinitely long stripe (l/w infinite) becomes:
Hshape,eff=(t1.Msat,1xe2x88x92t2.Msat,2)/wxe2x80x83xe2x80x83(2)
xe2x80x83where t1, (Msat,1) , and t2, (Msat,2) are the layer thicknesses (saturation magnetization) of the magnetic layers 1 and 2, respectively. The layer numbering is such that t1.Msat,1 greater than t2.Msat,2. Thus the effects of the two layers are of opposite sign and counteract each other, giving a compensating effect. It has been assumed that the anti-ferromagnetic coupling across the separation layer is very strong, as compared with the shape anisotropy field. In practice, this can be realized using, for example, subnanometer Ru or Rh layers. For a finite length 1, the shape anisotropy field is generally smaller, but the effect of compensation still applies.
3. Providing a sandwich structure comprising two magnetic layers with parallel easy magnetization axes due to the magnetocrystalline anisotropy and with opposite magnetization directions due to anti-ferromagnetic coupling by a non-magnetic layer 3 separating the two layers 1 and 2, while the magnetization directions due to the magnetocrystalline anisotropy for the two magnetic layers is directed perpendicularly to the longitudinal axis of the stripe leads to an increase of the effective magnetocrystalline anisotropy field HMCA, as compared with conventional devices, because the anisotropy energy has increased with the addition of a second layer, whereas the effective magnetization of the sandwich has decreased. Assuming, again, that the anti-ferromagnetic coupling across the separation layer is very strong, the proper expression for the effective magnetocrystalline anisotropy field HMCA becomes:
HMCA=2(t1.K1+t2.K2)/xcexc0(t1.Msat,1xe2x88x92t2.Msat,2)xe2x80x83xe2x80x83(3)
xe2x80x83where K1, and K2 are the anisotropy constants for layers 1 and 2, respectively.
The simultaneous occurrence of the latter two effects leads to a reduction of the total net effective anisotropy field because the easy axis direction due to the shape anisotropy field and the magnetocrystalline anisotropy field are perpendicular to each other. It is remarked that in general for a sensor the applied field to be measured is perpendicular to the direction of the effective anisotropy field, and that in the case of an MRAM in general the applied field used for switching must contain a component that is parallel to the anisotropy field. Preferably, the ratio of the shape anisotropy field Hshape,eff and the magnetocrystalline anisotropy field HMCA
Hshape,eff/HMCA (see equations 2 and 3)=0.5 xcexc0(t1.Msat,1xe2x88x92t2.Msat,2)2/(w.(t1.K1+t2.K2)xe2x80x83xe2x80x83(4)
lies between 0.5 and 1.5. The net effective anisotropy field is then greatly reduced.
As is apparent from the equations and will be exemplified by embodiments, the different functional dependences of the shape anisotropy field and the magnetocrystalline anisotropy field on the ratio of the thicknesses t1/t2 allow such conditions to be fulfilled by a proper choice of the ratio of thicknesses t1 and t2 or the ratio f t1.Msat,1/t2.Msat,2.
These and further aspects of the inventions will be apparent from and elucidated with reference to the embodiments described hereafter.